The invention relates to drilling in underground formations in the search for valuable materials such as oil and natural gas. In particular the invention relates to a method and means for controlling the course of a bore hole during drilling thereof.
In this specification and in the claims, the expression "the course of a bore hole" refers to the azimuth of the bore hole, being the direction of the bore hole with respect to the magnetic North Pole, as well as to the deviation of said bore hole, which is the direction of the bore hole with respect to the vertical.
While drilling a bore hole in underground formations, the bore hole tends to drift away from the desired course, as a result of the reaction of the drill bit and the drill string to the formations traversed, especially if such formations are dipping formations. The bore hole is regularly surveyed in order to determine the actual course thereof and the results of these surveys are used to decide whether the course of the bore hole needs to be corrected and to determine the extent of the corrections.
In order to correct the course of a bore hole that is being drilled, means for controlling the course of such bore hole are included in the drill string. Since these means are remotely controlled, they may be included permanently in the drill string. Examples of these means are the "variable bent sub" and the "orienting tool". The variable bent sub comprises a pipe section equipped with remotely controlled servo-mechanisms capable of controlling the degree of deflexion of the pipe section. An example of a variable bent sub is described in French patent application Ser. No. 2,175,620 (filed: 16th Mar. 1972; inventor: Russel, M. K.). The orienting tool comprises a housing and shoes that can be extended laterally with respect to the housing by means of remotely controlled servo-mechanisms. Further details of the orienting tool are given in U.S. Pat. No. 3,561,549 (filed: 7th June, 1968; issued: 9th Feb. 1971; inventors Garrison, E. P. and Tschirky, J. E.).
A major disadvantage of the above-mentioned steering means resides in their complexity and the cost of the servo-mechanisms thereof.
The object of the invention is a simple and reliable method and means for controlling the course of the bore hole.